Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 25,237,540	$ 13,894,348
Unrealized Gains	675,362	140,713
Unrealized Losses	(8,578)	(20,142)
Carrying Value	25,904,324	14,014,919
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,251,342	6,815,874
Unrealized Gains	295,952	82,267
Unrealized Losses	0	(4,131)
Carrying Value	4,547,294	6,894,010
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	150,000	100,000
Unrealized Gains	2,526	0
Unrealized Losses	0	(453)
Carrying Value	152,526	99,547
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,452	9,037
Unrealized Gains	0	1
Unrealized Losses	(3)	0
Carrying Value	23,449	9,038
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,569,724	4,109,372
Unrealized Gains	200,691	39,438
Unrealized Losses	(102)	(19)
Carrying Value	9,770,313	4,148,791
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,277,652	1,520,414
Unrealized Gains	45,809	17,929
Unrealized Losses	(7,488)	0
Carrying Value	7,315,973	1,538,343
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,965,370	1,339,651
Unrealized Gains	130,384	1,078
Unrealized Losses	(985)	(15,539)
Carrying Value	$ 4,094,769	$ 1,325,190

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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